An image forming apparatus of an electrophotographic system generally includes a fixing device that fixes a toner image, which is transferred to a sheet of paper, to the sheet of paper. The fixing device includes a heat roller and a press roller disposed so as to press the heat roller. The sheet of paper on which the toner image is transferred is conveyed between the heat roller and the press roller. Then, the toner image is fixed to the sheet of paper by heating and pressurizing the sheet of paper while the sheet of paper passes between the heat roller and the press roller. Accordingly, the image is formed on the recording sheet.
An example of such fixing device includes two thermistors, a first thermistor and a second thermistor, for detecting a surface temperature of the heat roller. The first thermistor is disposed so as to contact a center area of a surface of the heat roller in a width direction thereof. Then, a heater mounted in the heat roller is controlled so as to make a temperature detected by the first thermistor be a target temperature set in advance. The second thermistor is disposed so as to contact an end area of the surface of the heat roller in the width directions i.e., an area out of the contact area with a sheet of paper on the surface of the heat roller. For example, when image formations on sheets of papers with a relatively narrow width are repeatedly carried out successively, the surface temperature of the center area of the heat roller in the width direction is the target temperature, and in contrast thereto, heat at the end area of the heat roller in the width direction is not drawn by the recording sheets, which may lead to an anomalous heating condition due to the heat accumulation. The second thermistor is provided in order to detect its anomalous heating condition to interrupt heating of the heat roller by the heater (interrupt the image formations).
Further, extraneous matter (foreign matter) such as paper dust or toner adhered to the surface of the heat roller is transferred to the first thermistor, and the transferred extraneous matter is accumulated on the first thermistor, which may reduce a sensitivity of the first thermistor in some cases. If the sensitivity of the first thermistor is reduced, a temperature detected by the first thermistor becomes lower than an actual surface temperature of the heat roller. In this case, the heater continues to heat the heat roller even after an actual surface temperature of the heat roller has reached the target temperature. As a result, the heat roller may be in an anomalous heating condition.
In order to suppress these disadvantages, for example, JP-A-2005-173100 describes a following method including: starting warming-up of the heat roller by the heater in response to a power-on of the image forming apparatus: obtaining a difference between a temperature detected by the first thermistor and a temperature detected by the second thermistor during the warming-up; and compensating the temperature detected by the first thermistor based on the temperature difference (which shows an adhesion condition of extraneous matter on the first thermistor).